qemudomainsnapshotxml2xmltest: Allow regenerating into non-existing output file
qemudomainsnapshotxml2xmltest: Allow regenerating into non-existing output file

Replacing the 'virTestLoadFile' + 'virTestDifferenceFull' by '
virTestCompareToFile' allows to use the VIR_TEST_REGENERATE_OUTPUT=1
option to also generate the output file if it doesn't exist.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt